As is known in the art, one technique used to reduce NOx emissions from a diesel engine is to use a NOx storage catalyst or NOx trap. In a NOx storage catalyst, the NOx in the exhaust is absorbed on a catalytic surface. Unfortunately, the NOx trap gets polluted (or filled) by NOx after a period of time. When it is full, the NOx trap requires regeneration to consume the NOx. Regeneration means, in this context, that the exhaust composition is altered momentarily, i.e., the engine is run “rich”, i.e., with a surplus of fuel compared to the amount of oxygen that is available for the combustion. This results in large amounts of CO in the exhausts. The CO will enter the NOx storage catalyst, and react with the trapped NOx to form CO2 and N2.
There are, however, several problems connected to the regeneration process. For example, the regeneration process can contaminate the engine oil, since a part of the diesel fuel might hit the cylinder walls prior to being ignited. Once the diesel fuel has hit the cylinder walls, it will may absorbed in the thin oil film covering the cylinder walls, and eventually end up in the engine oil sump. If the oil in the sump is hot, some of the fuel will evaporate, hence leaving the oil. The evaporated fuel will eventually enter the engine intake through the oil vapor recovery system, and take part in a subsequent combustion, but the heavier fractions of the fuel may remain in the oil until the oil is changed. The fuel dilution of the engine oil is very detrimental to the oil quality. Of course, oil changes with close intervals will solve the problems with oil dilution, but this can be a very costly method; in a worst case scenario, the oil might be severely diluted after only a couple of thousands of kilometers, in less severe driving conditions, the oil might be acceptable after more than a hundred thousand kilometers. Closely spaced oil change intervals is therefore a very blunt way of ensuring a proper oil quality; it is unnecessary to change the oil often if the driving conditions are such that only few catalyst regenerations are necessary, and the oil often will reach temperatures allowing fuel evaporation.
One major problem with the oil dilution is that it is very complex; various regeneration strategies have different dilution effects, and the evaporation of fuel from the oil is very temperature dependent.
In the prior art addressing this problem, there are different approaches to this problem; in SAE 2002-01-1647, by T. Sagawa et al, the dilution process in a direct injected gasoline engine is studied. Gasoline is however quite different from diesel fuel, especially when it comes to evaporation characteristics.
SAE 2000-01-2838 and SAE 2000-01-1235, both by P. J. Shayler et al, also describe fuel dilution of the oil in direct injected gasoline engines.
XP 010257416 (ISBN 0-7803-3728-X) describes an onboard sensor for measuring the viscosity of engine oil. This sensor measures however only the viscosity of the oil. In a diesel engine, the viscosity will however remain quite unchanged, regardless of the fuel dilution level. Other oil characteristics like, e.g., the tribological characteristics, do however not remain the same with a diluted oil.
U.S. Pat. No. 5,169,785 describes a method for determining the fuel dilution of an oil by means of subjecting the oil for an ESR (electron spin resonance) spectrographic analysis. The method's basic principle is to measure the presence of vanadium in different molecule structures with different electron spin resonance. At present, this is regarded as a much too complicated and expensive method for on board vehicle use.
Finally, JP-A-7 098 168 describes a device for sensing the viscosity of engine oil. This device suffers from the same shortcomings as the device according to XP 010257416, namely that it does not measure the actual fuel dilution of the oil, but rather the viscosity drop emanating from the dilution. As previously stated, this makes the device less useful for diesel engines.
Another severe problem for many engine types (mainly on diesel engines and direct injected gasoline engines) is soot emissions. One technique used to reduce the emissions of soot is by means of soot filters. The soot filter filters out soot particles in the exhausts. However, after a while, the filter is full and needs regeneration. The regeneration process for a soot filter is very similar to the regeneration process for a NOx trap. There is however one major difference; the regeneration for the soot filter does not require an oxygen free environment. On the contrary, it is advantageous with oxygen in the exhausts, since the oxygen will react with the trapped soot particles and “post-combust” them into carbon dioxide (CO2) and water (H2O). One very critical demand on the exhausts for regeneration of soot is, however, the exhaust temperature; if the temperature is too low, the soot particles will not react with the oxygen in the exhaust.